/Engine/Source/Runtime/GeometryCore/Public/Parameterization/UVPacking.h
Uniformly scale all UV islands so that the largest fits in positive-unit-square, and translate each islands separately so that it's bbox-min is at the origin. So the islands are "stacked" and all fit in the unit box.
bool StackPack ( [IUVMeshView](API\Runtime\GeometryCore\Parameterization\FUVPacker\IUVMeshView) * Mesh, int NumIslands, [TFunctionRef](API\Runtime\Core\GenericPlatform\TFunctionRef)< void(int, [TArray](API\Runtime\Core\Containers\TArray)< int32 > &)> CopyIsland )